Images often contain many transitions. For instance, black and white and other dual tone images have transitions at the boundaries between their foreground features and their backgrounds, such as the transitions that demark line edges, font contours, and halftone dot patterns. Color images commonly include still additional transitions at the boundaries between differently colored foreground features. Consequently, the perceived quality of monotone and color prints tends to be strongly dependent upon the precision with which the printing process spatially positions these transitions.
In response to these technical challenges, template matching techniques have been proposed for more precisely controlling the size, positioning and number of picture elements ("pixels") that are printed on xerographic photoreceptors to render bitmapped images. For example, template matching has been developed for reducing the severity of certain printing artifacts, such as the observable stairstep-like scan structure (commonly referred to as "jaggies") that sometimes degrades the xerographically printed appearance of nonvertical and nonhorizontal lines. Examples of template matching techniques which may improve the output of images may be found in Tung, U.S. Pat. No. 4,847,641, "Piece-wise Print Image Enhancement for Dot Matrix Printers," issued Jul. 11, 1989, and Walsh et al., U.S. Pat. No. 4,437,122, "Low Resolution Raster Images," issued Mar. 13, 1984. Template matching effectively overcomes some of the sampling errors that are caused by the use of input data that is too coarse to accurately represent the higher spatial frequency content of the image.
However, in multiple color images, processing is generally performed on the pixels of a single color, called a color "separation." Standard template matching techniques, when applied to color separations, tend to introduce "separation errors." FIGS. 1-6 illustrate the problem of separation errors. FIG. 1 shows a portion 10 of a two-color image composed of "R" pixels (for example, red pixels) and "G" pixels (for example, green pixels). This image could further be composed of black pixels and other color or colors pixels. For the purposes of this application, black pixels may be considered colored pixels. In processing a color image, the colors are separated into multiple images, each of a single color, called "separations." FIG. 2 shows the G color separation 20 of image 10. The areas of image 10 which are G are separated into a separate image surrounded by blank pixels, shown in FIG. 2 as white pixels. For the purposes of this application, white pixels may be considered interchangeable with blank pixels. FIG. 3 shows the R color separation 30 of image 10.
FIG. 4 shows a smoothed output of G color separation 20 in an expanded pixel pattern such as might be produced by a template matching scheme such as described in Walsh et al. Traditional template matching techniques for smoothing images tend to fill in internal corners and round off protruding corners. FIG. 5 shows the smoothed output of R separation 30 in an expanded pixel pattern such as produced by Walsh et al.
When color separations are enhanced independently, even for images with no previous errors, the resulting combination of the separations may produce separation errors, particularly at corners and other contours.
FIG. 6 shows the result of recombining the two smoothed, expanded color separations of FIGS. 4 and 5. Although complimentary in most respects, it may be seen that there are errors at pixels 42-45, where the separations overlap, and at blank pixels 46 and 48, where neither color is represented.
Many of the ROS's (raster output scanners) that have been developed for xerographic printing employ a single beam or a multi-beam laser light source for supplying one or more intensity modulated light beams, together with a scanner (such as a polygon scanner) for cyclically deflecting the modulated laser beam or beams across a photoreceptor in a "fast scan direction" while the photoreceptor is being advanced simultaneously in an orthogonal "process direction." In practice, each of the laser beams typically is brought to focus on or near the photoreceptor surface to provide a substantially focused "scan spot." The scan spot, in turn, scans the photoreceptor in accordance with a predetermined scan pattern because the fast scan deflection of the laser beam or beams vectorally sums with the process direction motion of the photoreceptor. Indeed, the scan pattern is dependent upon and is determined by the scan rate (scan/sec) of the scanner, the spot size that is employed, and the process speed (inches/sec) of the photoreceptor. Such a scan pattern produces an exposure pattern because the scans are superpositioned on the photoreceptor, regardless of whether the scans simultaneously or sequentially expose the photoreceptor. Accordingly, it is to be understood that the present invention applies to printers and other display means that employ single beam or multi-beam ROS's, even though this disclosure features the single beam/single scan spot case for the sake of simplification.
It is an object of the present invention to enhance the contour resolution of features in multiple color images without creating separation errors in the output subpixels. In response to this object, a set of templates is provided which, when applied equally to each color, result in complimentary corrections. The problem of separation errors is solved by providing a single template set to be used to enhance each color separation of the image, the set including templates in which the second template is always the inverse of the first, and the third and fourth templates are 180.degree. rotations of the first two.